''Atlantic herring Clupea harengus
one of the most abundant species in the world''
A fish is a poikilothermic water-dwelling vertebrate with gills. Fishes (over 27,315 species) are a paraphyletic group and are divided into the bony fishes (class Osteichthyes, 22,000 species); cartilaginous fishes (class Chondrichthyes, 800 species); and various groups of jawless fishes (75 species), including lampreys and hagfish. Other aquatic animals, such as jellyfish and cuttlefish, are not true fish.
Fishes can be found in most all large bodies of water in either salt and fresh water, at depths ranging from just below the surface to several thousand meters. However, hyper-saline lakes like the Great Salt Lake do not support fish. Some species of fish have been specially bred to be kept and displayed in an aquarium.
Fish are an important source of food. Other water-dwelling animals such as mollusks and crustaceans (commonly called shellfish) are often considered as fish when used as food. Catching fish for the purpose of food or sport is known as fishing. The annual yield from all fisheries worldwide is about 100 million tonnes.
Overfishing is a threat to many species of fish. On May 15, 2003, the journal Nature reported that all large oceanic fish species worldwide had been so systematically overcaught that fewer than 10% of 1950 levels remained.  Particularly imperilled were sharks, Atlantic cod, and Pacific sardines. The authors recommended immediate, drastic cutbacks in fish catches and reservation of ocean habitats worldwide.
Black seabass Centropristis striata (photo: Uwe Kils)
See also: aquarium, freshwater aquarium fish species, marine aquarium fish species, deep sea fishes, list of fish families, list of fish common names, fish farming, fish anatomy, fish migration